This invention relates to a silicon nitride-cordierite composition, article, and process thereof.
Silicon nitride has been developed for use in applications where mechanical and thermal stresses require a material with strength, creep resistance, and low thermal expansion. The engineering/material properties of silicon nitride make it an ideal candidate for areas that place materials in extreme conditions. Engine applications are a typical example of the conditions where silicon nitride is being used. The use of silicon nitride composites for such application is described in C. L. Quackenbush, J. T. Neil, and J. T. Smith, Sintering Microstructure and Properties of Si.sub.3 n.sub.4 and siC Based Structural Ceramics, Paper No. 81-GT-220, Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for Presentation at the Gas Turbine Conference & Products Show, Mar. 9-12, 1981, Houston, Tex.
For silicon nitride, it has been shown that the development of a uniform SiO.sub.2 layer is effective in imparting improved oxidation corrosion resistance. The formation of such a layer is known to be dependent on the composition of the silicon nitride substrate and the specific conditions for its preparation. For example, if a part is given a heat treatment in an oxidizing atmosphere, the SiO.sub.2 layer is a function of the purity of the material and the temperature of the heat treatment. See S. I. Raider, R. Flitsch, J. A. Aboof, and W. A. Pliskin, Surface Oxidation of Silicon Nitride Films, in the Journal of the Electrochemical Society, Volume 123, No. 4, April 1976, pp. 560-565 and T. Hirai, K. Niihara, and T. Goto, oxidation of cvd Si.sub.3 N.sub.4 at 150 to 1650 deg C, Journal of the American Ceramic Society, Vol. 63, No. 7-8, July-August 1980, pp 419-423.
U.S. Pat. No. 4,542,109 to Pasto relates to a silicon nitride-cordierite article, and process for fabricating is described. The silicon nitride imparts a high mechanical strength, and the cordierite contributes to a lower thermal conductivity making the composition especially useful for internal parts of a diesel engine. The mechanical strength can be increased by crystallizing the continuous cordierite glassy phase of the article. Crystallization of the continuous cordierite glassy phase is accomplished by the addition of a nucleating agent such as zirconium dioxide in the formulation and a subsequent reheating step after densification.
Heretofore, prior art processes for forming silicon nitride-cordierite articles have not concentrated on improving the electrical resistance and corrosion resistance by a particular surface treatment.